/Engine/Plugins/Online/OnlineSubsystem/Source/Public/Interfaces/OnlineSessionInterface.h
Joins the session specified
true if the call completed successfully, false otherwise
bool JoinSession ( int32 LocalUserNum, [FName](API\Runtime\Core\UObject\FName) SessionName, const [FOnlineSessionSearchResult](API\Plugins\OnlineSubsystem\FOnlineSessionSearchResult) & DesiredSession )